My previously-working middleman installs seem to have succumbed to the usual bitrot, and have abruptly started throwing the error:
bundler: failed to load command: middleman (/Users/angus/.rvm/gems/ruby-2.6.5/bin/middleman)
NameError: uninitialized constant Haml::Options
  /Users/angus/.rvm/gems/ruby-2.6.5/gems/middleman-core-5.0.0.rc.1/lib/middleman-core/renderers/haml.rb:49:in `initialize'
  /Users/angus/.rvm/gems/ruby-2.6.5/gems/middleman-core-5.0.0.rc.1/lib/middleman-core/extension_manager.rb:70:in `new' 
(rest of stack trace snipped)
I’m seeing this running locally on MacOS with middleman-5.0.0rc1 and haml-6.0, and remotely on Ubuntu with the same combination (it also occurred with a much older version of middleman, 2.3.3).
Has anyone else run across this issue, or can suggest a fix? Thank you.